1. Field of Invention
The present invention relates to a metal-oxide semiconductor field effect transistor and a method for forming the same. More particularly, the present invention relates to a metal-oxide semiconductor field effect transistor having a strained layer and a method for forming the same.
2. Description of Related Art
Since the metal-oxide semiconductor field effect transistor possesses the superiority features such as, high reliability, low power consumption and low cost, it becomes the most important device in the integrated circuit. Typical metal-oxide semiconductor field effect transistor is formed on a silicon substrate and has a gate electrode, a source/drain region and a gate dielectric layer located between the gate electrode and the substrate. With the development of the electronic equipment such as telecommunication equipment, it is necessary to speed up the operation of the metal-oxide semiconductor field effect transistor. However, the application of the metal-oxide semiconductor field effect transistor is limited by the moving rate of the electrons and the holes within silicon.
Conventional, a metal-oxide semiconductor field effect transistor is provided to use germanium silicon epitaxy material to form the source/drain region so as to improve the moving rate of the electrons and the holes. Comparing to silicon, germanium possesses relatively smaller electron effective mass and hole effective mass so that the germanium silicon material can improve the electron migration probability and the hole migration probability. Moreover, since the lattice constant of germanium is larger than that of silicon, the germanium silicon epitaxy possesses a functionality as a strained layer. Therefore, the germanium silicon epitaxy further improves the efficiency of the metal-oxide semiconductor field effect transistor.
Nevertheless, it is easy for the metal-oxide semiconductor field effect transistor with the use of the germanium silicon epitaxy as the source/drain region to have structure defects. FIG. 1 is a cross-sectional view of a conventional metal-oxide semiconductor field effect transistor. As shown in FIG. 1, the metal-oxide semiconductor field effect transistor comprises a substrate 100, a gate electrode 102, a gate dielectric layer 104, a source/drain region 106 and a metal silicide layer 108. The metal silicide layer is the last part formed in the structure shown in FIG. 1. The method for forming the metal silicide layer 108 includes a self-aligned silicide process (salicide process). It is easy to generate spiking phenomenon at the border between the source/drain region 106 and the isolation structure 110 by using salicide process. Therefore, the metal silicide layer 108 is in contact with the substrate 100, which leads to junction leakage.